A multi-thread chain stitch sewing machine is equipped with one or more needles that rise and fall while holding needle threads and a looper that holds a looper thread and advances and retracts in a direction substantially orthogonal to the up-down movement pathways of the needles. The needles fall while passing through cloths to be sewn on a needle plate and rise so as to get out of the cloths to be sewn. The looper advances and retracts under the needle plate in synchronization with the rising and falling of the needles, and catches the loops of the needle threads (needle thread loops) held by the rising needles at the time of advance. The needles fall while passing through the cloths to be sewn and catch the looper thread held by the retracting looper.
The multi-thread chain stitch sewing machine repeats the above-mentioned operation to form seams on the cloths. FIGS. 1A and 1B are plan views showing seam structures of twin-needle multi-thread chain stitching, seen from the back side of sewn cloths. As shown in FIGS. 1A and 1B, seams of the multi-thread chain stitching are formed when needle thread loops formed by needle threads 20 and 20 on the back side of the sewn cloths are intertwined with a looper thread 10 in a form of inter-looping. In the general seam structure of multi-thread chain stitching shown in FIG. 1A, in the case that an end portion of a looper thread 10 that was cut at the end of sewing is pulled as indicated by the arrow shown in FIG. 1A, the looper thread 10 slips out of the last needle thread loops 20a and 20b formed by the needle threads 20 and 20. This slipping-out proceeds sequentially toward the sewing start side. As a result, there is a problem that ravel occurs in the whole seams.
Such ravel occurs similarly in seams of multi-thread chain stitching formed using three or more needle threads and a looper thread, and also occurs similarly in general sewing machines for forming a seam of multi-thread chain stitching, such as a covering chain stitch sewing machine.
Conventionally, various ravel preventing methods and various ravel preventing apparatuses for performing these methods have been proposed to prevent the above-mentioned occurrence of ravel peculiar to seams of multi-thread chain stitching. As one of the proposals, a ravel preventing method and a ravel preventing apparatus proposed by the present applicant are available (see Japanese Patent No. 2879399, for example). In this ravel preventing method, a looper thread hook is provided to hold a looper thread that was passed through needle thread loops by the advance of a looper at the advance end portion of the looper. The looper thread hook is operated in a state in which the needles have risen and the looper has advanced at the end of usual sewing. In a state in which the looper thread is held by the looper thread hook, sewing for one stitch is performed, and then the needle threads and the looper thread are cut.
With this method, during sewing for one stitch the looper thread 10 held by the looper thread hook is intertwined with the last needle thread loops 20a and 20b formed by the needle threads 20 and 20 in a form of interlacing, and the seams as shown in FIG. 1B are formed. The looper thread 10 intertwined in this manner cannot slip out of the last needle thread loops 20a and 20b even if an end portion thereof is pulled as indicated by the arrow. As a result, seam ravel can be prevented at the stage of occurrence.